This invention relates to a method for recovering oil from a subterranean oil-bearing formation. The method employs surfactants and additions to such surfactants.
The petroleum industry has recognized for many years that the natural formation energy of a petroleum reservoir will produce only a portion of the crude oil originally in the reservoir. Waterflooding has been commonly used to enhance the recovery. In waterflooding, water, or other aqueous fluid, is introduced through injection wells to drive oil through the reservoir to offset producing wells. Such flooding, however, is inefficient, leaving behind in the reservoir more than half of the original reservoir crude oil. The reason for this is that much of the oil that is retained after a typical waterflood is in the form of discontinuous globules or discrete droplets which are trapped within the pore spaces of the reservoir. The normal interfacial tension between the reservoir oil and water used for flooding is so high that these discrete droplets are unable to deform sufficiently to pass through narrow constrictions in the pore channels of the formation.
Surface-active agents or surfactants have been found to reduce or lower the interfacial tension between the water and reservoir oil so that the oil droplets may deform, coalesce and flow with the flood water toward producing wells.
A number of various types of surfactants have been suggested for use--anionic surfactants, cationic surfactants, and nonionic surfactants. Anionic surfactants have generally been preferred.
However, a problem with using any surfactant in waterflooding processes for the recovery of oil is that the efficiency of the oil displacement is strongly affected by the surfactant's surface activity, or the extent of the surfactant's lowering of the oil-water interfacial tension. Many surfactants are unable to exhibit high surface activity at temperatures as high as or higher than about 120 degrees Fahrenheit and/or in aqueous solutions containing concentrations of inorganic salts as high as or higher than 2 percent sodium chloride (NaCl) and over 0.2 percent calcium chloride (CaCl.sub.2) and magnesium chloride (MgCl.sub.2). Many oilfield reservoirs have such high temperatures and inorganic salt concentrations. In fact, it would not be unusual for reservoir brine to have concentrations of over 6 or even 8 percent NaCl and over 0.6 or even 0.8 percent CaCl.sub.2 and MgCl.sub.2. For example, West Texas (Permian Basin) and Gulf Coast reservoirs typically have about 100,000 parts per million total dissolved solids comprised of about 10 percent NaCl and about 0.1 percent other salts such as CaCl.sub.2 and MgCl.sub.2. (All percentages reported herein are percents by weight unless otherwise noted). Other problems with surfactant flooding processes include poor phase behavior of the flooding medium upon flow through a formation and a generally lower ability to displace oil as the temperature of the formation increases.
Surfactants that can retain high surface activity in reservoir conditions have been sought and a number have been suggested. Such surfactants have included blends of surfactants or surfactant combinations. One example is disclosed in U.S. Pat. No. 3,939,911, issued Feb. 24, 1976 to Maddox, Jr., claiming a three surfactant system. The three surfactant system includes an anionic surfactant, a phosphate ester surfactant, and a particular sulfonated betaine which is an amphoteric surfactant.
Others have disclosed the use of certain amphoteric surfactants in surfactant flooding processes. For example, U.S. Pat. No. 4,076,743 issued Feb. 28, 1978 to Koch et al, U.S. Pat. No. 4,216,097 issued Aug. 5, 1980 to Stournas, U.S. Pat. No. 4,438,045 issued Mar. 20, 1984 to Nieh, and U.S. Pat. No. 4,502,540 issued Mar. 5, 1985 to Byham, all disclose the use of certain amphoteric surfactants in surfactant flooding processes. Some of the patents report the ability of the surfactants to maintain surface activity in the presence of relatively high amounts of divalent ions or the compatibility of the surfactants with such ions.
However, there continues to be a need for additional and improved surfactants having utility in enhanced oil recovery operations and for more efficient methods of recovering oil from subterranean formations.